[Amps] SB 200 Puzzler

Eg9k at aol.com Eg9k at aol.com
Fri Feb 11 20:47:17 EST 2005


Hi Everbody  -  Name here is Harold, located in southern Indiana. I recently 
picked up a Heathkit SB 200 ( presently have a SB 201 which works fine ). I 
have owned a couple of SB 200's in the past and have been able to troubleshoot  
and fix any problems that I had with them. I have home brewed about 5-6 
linears and have owned about 30 different amps over the past 45 years, so I'm no 
stranger to working on them." BUT " I've run into this problem with this recently 
aquired SB 200 thats has me somewhat puzzled, been reading some of the Amp 
posts and the replys, and hoped someone might be of some help. As to the amp, 
the supply voltage is 2400v no load & about 2200v under load, it tunes and loads 
up fine ( with the tune knob indicator arrow in the correct area ) on 10,15 
and 20 meters, putting out between 500-600 watts with about 70 watts of drive 
from a MK5 Yaesu. It also will tune and load up to full output on 40 and 75 
meters, "BUT" the tune indicator knob arrow on the front panel is almost in the 
20 mtr.area for both bands, as is the plate tuning capacitor plates ( both 
capacitor have  plates are set correctly - fully meshed, max capacity with dial 
arrow set at the 9 oclock position ). It appears to me that It's picked up some 
added capacity on 40 and 75
meter positions and thats the reason the plate tuning capacitor is tuning to 
less capacity ( 20 mtr.area ) for resonance in the 40 and 75 meter bands. I've 
checked about everthing, I've cleaned (ink eraser) and completely checked the 
band switch (continunity), removed and checked the values of the plate 
blocking cap C24, 1000uuf, C26, 100uufd  parallel cap, and the 500uufd switched in 
loading cap for 75 mtrs. All were within 10% on my cap. meter. I lifted the 1.1 
mhy. RF choke and output lead to check the loading cap. tested perfect. With 
the plate tuning cap fully meshed ( max. capacity) attached to the total turns 
of the coil assy. the capacity reading was 215 uufd, with the plates fully 
open the reading was  65 uufd verifying that the plate tuning cap was at it's 
proper value of 150 uufd. My questions are ---
1. Is 65 uufd a reasonable amount of stray capacitance for just the final 
coil assy ?
2. Does anyone know if there is a minimum capacitance  value for the plate 
tuning cap ?
3. Any suggestions / help would be much appreciated.

THANKS for reading   -    Best Regards   -   Harold    -     K9ge

PS: Also checked relay/contacts  (use external keying relay also).
      Also changed tubes when original problem occured - same problem
     Also checked ( changed ) coax between radio and amp.
     Also adjusted input tuned ckt. coils for min. swr on all bands.
     Also checked & re-checked all solder joints involved
     Also used two different dummy loads for testing ( MFJ Versaload & Heath 
Cantenna )


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